This invention relates to a method of producing carbon with electrically active sites.
Diamond is well-appreciated as an excellent electrical insulator. However, a rare class of diamond is found in nature, codified as Type IIb, which has p-type semiconducting properties. Research by one of the inventors (Ref. Sellschop J P F et al, Int J of App Rad and Isot. 28(1977)277) demonstrated that this was due to the presence of boron in the diamond.
The importance of diamond as a semiconducting material has long been anticipated, arising from the many unique physical properties of this material that would render diamond as a material of singular importance in electronic applications, including in rugged environments.
That this has not yet been realised is due to the difficulties in getting this type of boron-doped diamond with a sufficiently low density of defects, and intrinsic and irradiation-induced defects and sufficient spatial homogeneity of the dopant throughout the diamond. Type IIb diamonds are extremely rare in nature, but have been produced synthetically both in high pressure, high temperature growth (HPHT), and in chemical vapour deposition (CVD) growth, by the addition of boron to the synthesis mix. These successes are far from ideal, and do not solve the need, since they may be expensive and slow in the HPHT case and hard to control quantitatively in both cases; homogeneity is hard to achieve. Large defect-free crystals are hard to achieve by the HPHT method, and the CVD method (other than in special circumstances, such as using diamond itself as a growth substrate) produces polycrystalline materials.
This has given rise to a strong thrust to achieve doping by the introduction of relevant materials, such as boron, by a technique known as ion implantation. In this manner, p-type doping has been claimed to have been achieved. There are major difficulties inextricably associated with this technique, however, and one of the most serious of these is that of the radiation damage caused by the penetrating boron ion. Another very serious problem is that the characteristic features of the implantation profile are highly inhomogeneous with regard to the overall geometry of the sample, and there is no evident solution to this feature, even if implantations over a range of different energies are made. In regard to the radiation damage, various temperature regimes and sequences have been used in an effort to restore to some degree the integrity of the damaged crystal lattice, to reduce the number of damage sites which would act as traps and to enhance the probability of providing substitutional sites for the dopant ion in the hope that it will then preferentially populate such substitutional sites. Furthermore, ion implantation is normally automatically considered as having a geometry where the accelerated ion beam addresses the sample through a flat surface. It cannot handle samples of random and various shapes in a sensible way.
According to the invention, a method of producing carbon with electrically active sites includes the steps of providing a source of carbon and exposing that source to irradiation of an energy suitable to cause the photonuclear transmutation of some of the carbon atoms into boron.
The carbon source may be any allotrope of carbon including diamond, diamond-like materials, amorphous carbon, graphite, carbon nanostructures or fullerenes. The invention provides a method of producing a population of electrically active sites, some of which will be substitutional when the carbon has a crystalline structure, by the homogeneous photonuclear transmutation of some of the carbon atoms into boron. The transmutation may be assisted and enhanced if appropriate by one or more of a selection of annealing regimes: thermal heating and/or electron beam heating or any other form of specimen-specific heating, either post-irradiation or during irradiation; laser irradiation again either post irradiation or during irradiation, assisted if necessary simultaneously by thermal or electron beam heating; laser illumination at specifically selected wavelengths and/or of wavelength bands, again either post-irradiation or during irradiation or both, assisted if necessary by sample heating of thermal or electron beam origin or other means: including the concept of resonant effects in the annealing process including specifically resonant laser annealing at room or elevated temperatures, including also specifically combinations of temperature protocols such as low temperature irradiation followed by rapid thermal annealing.
The invention has particular application to the controlled and homogeneous doping of diamonds of all types, shapes and sizes, single crystal and polycrystalline, natural and synthetic. The synthetic diamond may be produced by high pressure/high temperature growth or chemical vapour deposition.
The irradiation will preferably be achieved using photons, and particularly gamma rays, but may also be achieved by using other irradiation sources such as electrons.
The interaction of photons with matter is a gentle one in so far as radiation damage is concerned in comparison with that of charged particles or neutrons. This interaction takes place through the mechanisms of the photoelectric effect, of Compton scattering, and of pair production. It is important to note that all three of these mechanisms are electromagnetic in origin, rather than operating through the nuclear interaction. Hence the disruption to the ordered crystal lattice is minimal, and particularly so as compared with that caused inherently by charged particles or neutrons.
Where radiation damage is caused, for example by an energetic proton or neutron and a recoiling boron being produced, such damage may be reduced by use of one or other of the annealing methods described above. Photons have a high penetrating power as compared with all other typical radiations, hence lending themselves to an extremely high degree of homogeneity in any effects which they produce.
It is important that the energy of the radiation is chosen so that the desired photonuclear reaction leading to the formation of boron is achieved. The minimum energy of the radiation necessary to achieve a particular photonuclear reaction will vary according to the specific energetics of the reaction. Examples are provided hereinafter. Typically, the energy of the radiation will be in the range 16 MeV to 32 MeV.
It is further preferred that the energy of the radiation is chosen to excite the giant dipole resonance (GDR) which leads to an enhancement of the boron production rate. The GDR is a broad resonance and bremsstrahlung can be produced by means of an electron accelerator such that the endpoint energy of the bremsstrahlung spectrum is above the region of the GDR providing thereby photons in the relevant energy range to excite the GDR. Certain advantages may be achieved by the use of monoenergetic (monochromatic) photons of selected energy, or by a defined window of photon energies of chosen energy width and median energy.
The photonuclear reaction can be employed to effect the transmutation of carbon atoms to boron atoms with complete control of the number of boron atoms produced. Doping concentrations of a few parts of boron per million carbon atoms, can be achieved with the ability of producing smaller or larger dopant concentrations.